You may want to try in Template:Sandbox , Click "Edit source" button and click "Visual editing is not available here" button ......
Topic on 2017 wikitext editor/Feedback
/me using Chromium 56 on Arch Linux .
Thanks, I reported this as https://phabricator.wikimedia.org/T159834 . I don't get to decide the priorities, but since there is potential for bad edits, I tried to flag this "leaking" as something they should look into ASAP.
This fix is being deployed this week; it will reach the Wikipedias on Thursday.
Although allegedly that didn't fix the problem, they're working on this.